I. Field of the Invention
The present invention relates generally to the field of cardiac rhythm/function management devices, and more particularly relates to cardiac rhythm/function management devices that automatically determine whether or not a pacing stimulus to the atrium(s) and/or ventricle(s) results in capture. The cardiac device of the present invention is suitable for use in either a unipolar or bipolar pacing/sensing configuration and may be utilized to verify either atrial or ventricular capture in a heart having either normal or abnormal intrinsic conduction.
II. Background of the Prior Art
In a normal heart, the sino-atrial (SA) node initiates the myocardial stimulation of the atrium. The SA node comprises a bundle of unique cells disposed within the roof of the right atrium. The SA node cells are in electrical communication with the surrounding atrial muscle cells such that the depolarization of the SA node cells causes the adjacent atrial muscle cells to depolarize. The depolarization causes the atria to contract forcing blood into the ventricles. The depolarization of the SA node is further communicated to the atrio-ventricular (AV) node. The AV node communicates the depolarization impulse to the ventricles sequentially through the Bundle of His and Purkinje fibers. The time for the depolarization impulse to travel from the AV node through the Bundle of His and Purkinje fibers results in a brief delay for ventricular contraction. Therefore, ventricular contraction or systole lags behind atrial systole. The sequential contraction of the atria and ventricles allows the atria to fill the ventricles before the ventricles pump the blood through the body and lungs. Atrial and ventricular diastole follow wherein the heart muscle or myocardium is re-polarized and relaxed prior to the next contraction.
When the intrinsic stimulation system fails or functions abnormally an implanted pacing device may be needed to deliver an electrical (pacing) stimulus to the heart. When the strength of the stimulus is sufficient, the artificial electrical stimulus can cause the muscle cells surrounding the electrode to depolarize. This depolarization will spread out through the entire chamber or chambers of the heart and result in contractions. Thus, electrical stimulation, when applied at the appropriate time and location, can maintain the proper heart rate and/or efficient contraction. This typically is the purpose of a cardiac rhythm/function management device. Further, certain conditions result in heart fibrillation and require a significant electrical stimulus to defibrillate the heart.
Cardiac rhythm/function management devices are widely used for supplanting the heart""s natural pacing functions and for defibrillating the heart. The devices may be used to correct various abnormalities, including total or partial heart block, arrhythmias, myocardial infarctions, congestive heart failure, congenital heart disorders, and various other rhythm disturbances within the heart. A cardiac rhythm/function management device typically includes a pulse generator to generate an electrical stimulus and at least one lead to transfer the electrical stimulus to the heart. The electrical stimulus or pacing stimulus may be directed to one or more of the atria and/or ventricles. Further, the leads may also be used to sense for electrical impulses in one or more of the atria and/or the ventricles. A ventricular lead of the cardiac rhythm/function management device may also be used in a defibrillation mode to defibrillate the heart. The cardiac rhythm/function management device typically includes a pacing output circuit designed to selectively deliver stimulus pulses through the lead to one or more electrodes. The pacing output circuit includes a power supply, switches, a pacing charge storage capacitor, and a coupling capacitor, all of which cooperatively operate under the direction of a controller to perform a charging cycle, a pacing cycle, and a recharging cycle.
Regardless of the particular device""s configuration (ie: ventricular pacing, atrial pacing, multi-chamber pacing, etc.), cardiac rhythm/function management devices generally operate by stimulating the muscle cells adjacent to the pacing electrode or set of electrodes. The devices provide one or more particular stimuli to the heart that overcomes the abnormality and/or confers an appropriate rhythm. When the strength of the pacing stimulus meets or exceeds a threshold level, the resulting depolarization propagates through the heart. A pacing stimulus that initiates a propagated depolarization is said to have xe2x80x9ccapturedxe2x80x9d the heart.
Thus, the success of a pacing stimulus in capturing the heart depends on whether or not the current of the pacing stimulus to the myocardium exceeds the threshold value. The threshold value, frequently referred to as the capture threshold, is related to the electrical field intensity required to alter the permeability of the myocardial cells to thereby initiate cell depolarization. If the local electrical field associated with the pacing stimulus does not exceed the capture threshold, then the permeability of the myocardial cells will not be altered sufficiently to initiate depolarization. If, on the other hand, the local electrical field associated with the pacing stimulus exceeds the capture threshold, then myocardial cell permeability will be sufficiently altered to propagate depolarization.
The capture threshold may vary over time. Changes in the capture threshold may be detected by monitoring the efficacy of stimulating pulses at a given energy level. If capture does not occur at a particular stimulation energy level which previously was adequate to effect capture, then it can be surmised that the capture threshold has increased and that the stimulation energy should be increased. On the other hand, if capture occurs consistently at a particular stimulation energy level over a relatively large number of successive stimulation cycles, then it is possible that the capture threshold has decreased such that the stimulation energy is being delivered at a level higher than necessary to effect capture.
The ability of a pacing device to detect capture is desirable in that delivering stimulation pulses having energy far in excess of the patient""s capture threshold is wasteful of the limited power supply. In order to minimize current drain on the power supply, it is desirable to automatically adjust the device to deliver the lowest energy level that will reliably capture the heart. To accomplish this, a process known as xe2x80x9ccapture verificationxe2x80x9d must be performed wherein the device monitors to determine whether an evoked depolarization occurs in the pre-selected heart chamber following the delivery of each pacing stimulus pulse to the chamber.
In many cardiac rhythm/function management devices, the device does not determine whether or not a pacing stimulus or set of stimuli have promoted the heart to contract. Efforts have been made to develop a cardiac rhythm/function management device that verifies capture. For example, special sensing amplifiers and algorithms have been added to the device to detect evoked potential presented in an electrode after a stimulus is delivered to that electrode. However, it has been found that such capture verification is difficult due to polarization voltages or xe2x80x9cafter-potentialxe2x80x9d which develop at the heart tissue/electrode interface following the application of the stimulation pulses.
The ability to verify capture is further affected by other variables, including patient activity, body position, drugs, lead movement, noise, etc. Because of the multiplicity of variables, the algorithms used to determine capture are frequently complex. The complexity adds to the costs and the likelihood of errors in the software. Also, adding specialized components to verify capture increases the cost and complexity of the pacing apparatus. Therefore, a need exists for an apparatus for sensing ventricular capture that does not require specialized components, such as sense amplifiers, to detect ventricular activities. Further, there is a need for an apparatus and method for verifying ventricular capture that does not require sophisticated signal processing.
U.S. Pat. No. 5,222,493 issued to Sholder (the ""493 patent) describes a cardiac rhythm management device having a capture verification circuit that senses evoked potential between a pacing electrode and an additional indifferent electrode. Sholder describes positioning the additional indifferent electrode on the front or back of the header of the pacemaker or, alternatively, on an additional lead or an existing pacing lead. Although an indifferent electrode is less affected by saturation voltages, the capture verification circuit described by Sholder requires direct input from the stimulating electrode. Thus, the electrical charges that build up around the stimulating electrode after delivery of a pacing pulse affects the accurate determination of whether the pacing results in capture. Additionally, Sholder does not describe a verification circuit for capture verification of biventricular or other multi-site stimulation modes. Further, the requirement of an indifferent electrode and either a switching circuit or an additional sensing amplifier may increase the costs thereof
U.S. Pat. No. 6,148,234 issued to Struble (the ""234 patent) describes a cardiac rhythm management system having a form of capture verification. The device described in the ""234 patent includes a two electrode, biventricular pacing system that verifies non-capture (from one of the two electrodes) by determining whether there is conducted depolarization on any electrode. The concept is that when one of the two stimulating electrodes captures while the other fails, the depolarization that is induced by the capturing electrode will conduct to the location of the non-capturing electrode and be detected by the latter electrode. Although detection of this xe2x80x9cconductedxe2x80x9d depolarization from a non-capturing electrode may be easier than detection of an xe2x80x9cevokedxe2x80x9d depolarization from a capturing electrode, Struble does not describe a device fully operable when an AV block is present, and further does not describe a device having a single electrode mode or three or more electrode mode. Thus, there is a need for an apparatus and method which is less affected by post-stimulation artifacts, more versatile and reliable to verify both capture and non-capture utilizing different stimulation configurations (for example, single or multiple electrode stimulation) wherein various heart conditions are present (for example, normal or abnormal AV conduction).
The present invention meets the above needs and provides additional improvements and advantages that will be recognized by those skilled in the art upon review of the following figures and description.
The present invention provides a device and method for verifying capture which looks for propagated depolarization originating from heart muscle cells in distant locations that spread or propagate towards a detecting electrode. The device and method of the present invention confirms capture if a propagated depolarization is detected at a non-stimulating electrode and for the same cardiac cycle, no propagated depolarization is detected at any stimulating electrodes shortly after a stimulation. By checking for propagated depolarization from all available electrodes, both stimulating and non-stimulating, the present invention is able to verify capture (or non-capture) with a variety of stimulation configurations (single or multiple electrode) and under various patient conditions (normal or abnormal intrinsic conduction). The present invention does not require special sense amplifiers and complex algorithms, thereby simplifying implementation and reducing costs.
The cardiac electrical stimulation system of the present invention has an autocapture stimulation/sensing configuration for use in the atrium or ventricles and generally includes one or more leads, a pulse generator, a sensing circuit and a control unit. The leads have one or more stimulating electrodes and one or more passive (non-stimulating) electrodes and may be positioned in the atrium or ventricles of the heart. The pulse generator is electrically coupled to the stimulating electrodes for providing an electrical stimulus to at least one of the atrium and ventricles of the heart. The sensing circuit is electrically coupled to the stimulating electrodes and the passive electrodes, wherein the sensing circuit senses a response by the heart to the electrical stimulus.
The sensing circuit includes a timing circuit, a first detection circuit coupled to the stimulating electrodes, and a second detection circuit coupled to the passive electrodes. Capture of the electrical stimulus is confirmed or verified if the second detection circuit detects depolarization but the first detection circuit does not detect depolarization during a predetermined detection interval. Non-capture of the electrical stimulus by the heart is confirmed if the first detection circuit detects depolarization during the predetermined detection window or no depolarization is detected by either detection circuit during the predetermined detection window.
In use, after the leads, having the stimulating and passive electrodes, are in the desired position within the heart, the pulse generator delivers an electrical stimulus to one or more stimulating electrode(s). The system then senses for depolarization at both the stimulating and passive electrodes during the predetermined detection window. The timing interval of the predetermined detection window may be adjusted to exclude sensing of stimulus artifacts. If capture of the electrical stimulus by the heart is not confirmed, the amplitude of the electrical stimulus may be increased.
The control unit configures the electrodes by designating them as stimulation or passive, controls the delivery of stimuli, determines the detection window, coordinates the execution of capturation verification, and changes amplitude of the stimulus according to the results of capture verification.
These and other advantages of the present invention will become readily apparent to those skilled in the art from a review of the following detailed description of the preferred embodiment especially when considered in conjunction with the claims and accompanying drawings in which like numerals in the several views refer to corresponding parts.